The present invention relates to a linear actuator having a screw shaft driven by a motor and a screw nut converting rotary motion of the screw shaft into linear motion.
Japanese Utility Model Publication No. 61-3250 discloses a linear Actuator, which includes a screw shaft, a screw nut threaded with the screw shaft, a rod attached to the nut and an electric motor for rotating the screw shaft in the forward and reverse directions. When the electric motor is driven to rotate in the forward or the reverse direction, the screw shaft rotates to thereby cause the nut to move forward or backward along the axis of the shaft together with the rod.
The linear actuator of this type requires setting of the stroke of the rod. In the disclosed linear actuator, sensors such as micro-switches or proximity switches are provided to detect the displacement of the nut such that when the nut has displaced a predetermined distance, a sensor detects arrival of the nut and cuts off the motor current.
To cut off the motor current, an excess current detecting printed circuit board may be used. The printed circuit board of this type is provided essentially for the protection of peripheral parts against damage when the nut or the rod overruns a preset extreme position. Accordingly, detection with a sensor of the arrival of the nut or the rod at the extreme position is a prerequisite condition.
In order to set the stroke of the rod by using sensors, it is necessary to install sensors, such as micro-switches or proximity switches, inside the linear actuator by means of screws. In connection therewith, cases for the sensors, waterproof packings, wiring between the sensors and the electric motor, and a waterproofing treatment are also needed. This increases parts variety and assembling man-hours.
In the case where the nut or the rod is stopped moving at the abutment or collision with a stopper when it reaches one of the two extreme positions, an impact is created, which may damage the parts and produce a noise. Furthermore, due to the inertia, the nut or the rod does not stop immediately, tending to cause an operation failure of the linear actuator due to the seizing between the screw shaft and the nut.
It is accordingly an object of the present invention to provide a linear actuator with abutment stoppers, which is capable of reducing an impact and a noise produced when the nut or the rod reaches one of two preset extreme positions.
Another object of the present invention is to provide a linear actuator, which is able to prevent the screw shaft and the nut from becoming seized together when the nut or the rod is stopped moving at a collision or abutment with the stopper.
A further object of the present invention is to provide a linear actuator, which is capable of stopping the nut or the rod at a given intermediate portion additional to the two present extreme positions.
To achieve the foregoing object, according to the present invention, there is provided a linear actuator comprising: a housing; a screw shaft rotatably supported within the housing with play in motion in the axial direction of the screw shaft; a screw nut threaded with the screw shaft and converting rotary motion of the screw shaft into linear motion; a rod attached to the screw nut for movement in unison with the screw nut; an electric motor rotating the screw shaft in the forward and reverse directions; a cushioning stopper disposed between the housing and the screw shaft and elastically deformable to absorb a force acting from the screw shaft to the housing when the screw shaft moves relative to the housing in the axial direction within the range of the play; a power supply providing electric power to the motor; and an overload detecting device disposed between the power supply and the motor and cutting off the supply of power from the power supply to the motor when the overload detecting device detects excessive current in the motor.
With this arrangement, when the motor is driven to rotate in the forward or the reverse direction, the nut threaded with the screw shaft moves linearly along the axis of screw shaft to thereby extend or contract the rod relative to the housing. When the rod is stopped moving upon collision or abutment with an obstacle or stopper at one of the two preset extreme positions or a given position intermediate between the extreme positions, the screw shaft is subjected to a reaction force. The screw shaft, which is allowed to move in the axial direction within the range of the play, is displaced in the axial direction by the reaction force while forcing the cushioning stopper against the housing. Thus, the cushioning stopper undergoes elastic deformation and thereby suppresses an impact force and noise produced when the rod abuts on the obstacle or stopper.
When the rod is stopped moving upon abutment with the obstacle or stopper, excessive current flows in the motor. The overload-detecting device disposed between the power supply and the motor detects the excessive motor current. The overload detecting device comprises an excessive current detecting printed circuit board. The overload detecting device cuts off the supply of power to the motor when the elastic deformation of the cushioning stopper increases to a predetermined degree. By thus linking the operation of the overload detecting device and the elastic deformation of the cushioning stopper, it is possible to lower the peak current in the motor as compared to a linear actuator having a rigid structure. With the use of the elastically deformable cushioning stopper, motor current increases with a gentle gradient. This ensures that the excessive motor current with lowered peak value is detected and the supply of power to the motor is cut off at early stages of current increase.